The present invention relates to catalyst handling in a process for converting oxygenates to an olefin product. The present invention relates generally to a method of catalyst conservation in an oxygenate-to-olefin (OTO) process utilizing a fluidized oxygenate conversion zone and a relatively expensive catalyst and the use of a wet scrubbing step that recovers these contaminating catalyst particles in a scrubbing liquid which has organic bases or caustics added thereto to prevent a buildup of acid that adversely affects the catalyst.
The worldwide petrochemical industry is concerned with the production of light olefin materials such as ethylene and propylene for use in the production of numerous important chemical products. The main source for these materials is the steam cracking of petroleum feeds. The industry has long sought a source other than petroleum for the raw materials needed to supply the demand light olefin materials. The prior art has focused on different procedures for catalytically converting oxygenates such as methanol into the desired light olefin products. The major focus of routes to produce these desired light olefins has been on methanol conversion technology.
U.S. Pat. No. 6,403,854; U.S. Pat. No. 6,166,282 and U.S. Pat. No. 5,744,680 point to the use of a fluidized reaction zone along with a fluidized regeneration zone as the preferred commercial solution to the problem of effectively and efficiently using an ELAPO or SAPO-type of catalyst system in OTO service. The use of this technology gives rise to a substantial problem of solid-vapor separation to efficiently separate the particles of the fluidized catalyst from the vapor components exiting the OTO conversion zone. U.S. Pat. No. 6,166,282 shows a series of three cyclonic separation means to separate spent OTO catalyst from the product effluent stream. There still remains a very substantial problem of OTO or catalyst contamination of the product effluent stream withdrawn from the fluidized conversion zone.
U.S. Pat. No. 5,744,680 discloses the use of a wet scrubbing step on the cooled effluent stream from an OTO conversion zone to remove ELAPO molecular sieve-containing catalyst particles from this effluent stream but merely teaches the withdrawal of the catalyst-containing bottom stream from the wet scrubbing step for further unspecified treatment. U.S. Pat. No. 6,121,504 uses wet scrubbing to quench the effluent stream from the OTO conversion zone and produce a bottom stream which is recirculated to the wet scrubbing stream except for a drag stream that enters a stripping zone for purposes of heat recovery. U.S. Pat. No. 6,403,854 exemplifies a quench arrangement for the hot effluent stream recovered from the OTO conversion zone with first stage that removes catalyst fines entrained in the product effluent stream. U.S. Pat. No. 6,870,072 discloses the problem of product effluent contamination with catalyst particles and uses a wet scrubbing zone to remove these contaminating particles but no means of recovery and reuse of the catalyst particles. US 2004/0064006A1 discloses a process for efficient handling of catalyst fines. However, this published patent application does not recognize the problems that would be caused within that process with the buildup of acids that would adversely effect the catalyst.
A substantial economic problem for the OTO process is the amount of fresh catalyst that must be added to the OTO or fluidized conversion zone in order to maintain the catalyst inventory in the OTO conversion system when the product effluent stream contains substantial amounts of contaminating catalyst particles. This problem of effluent contamination by catalyst particles is because of the relatively high expense ELAPO or SAPO molecular sieves. This invention addresses the problem of reducing the loss of catalyst particles from a fluidized OTO conversion zone to decrease the consumption of relatively expensive catalyst thereby improving the economics of the OTO conversion process. The preferred oxygenate to olefin conversion process is generally referred to as a methanol-to-olefin(s), or MTO process, where methanol, is converted in a reactor to primarily ethylene and/or propylene in the presence of a catalyst—typically a molecular sieve catalyst made from a molecular sieve catalyst composition. This oxygenate to olefin reaction uses a catalyst that is maintained under operating conditions with carbonaceous deposits thereon. The carbonaceous deposits are often referred to as coke. Catalyst, for the purpose herein, is classified according to the size of the catalyst. Catalyst particles are larger than catalyst fines. Catalysts particles are typically retained in the reactor by the particle size separators that disengage or separate the catalyst particles from the effluent stream, which effluent stream passes through the particle size separators into the product recovery train. Catalyst fines are carried into the effluent stream.
Typically, catalyst particles above 40 microns in size are added to the reactor to catalyze a reaction. During the reaction, the catalyst develops carbonaceous deposits. Withdrawing a portion of the catalyst from the reactor and burning the carbonaceous deposits off of the catalyst particles controls the aggregate amount of the carbonaceous deposits on catalyst in the reactor. As the catalyst particles travel through the reactor, they break down into smaller particles due to contact between catalyst particles as well as with the various parts of the reactor. As they break down in size, they eventually become catalyst fines. Particle size separators, such as cyclones, are placed in the reactors and regenerators to retain useful catalyst particles in the reactor/regenerator system. Catalyst fines (typically less than 40 microns and more typically less than 20 microns) are generally not retained by the particle size separators and leave the regenerator through the flue. Catalyst fines in the reactor become carried into the effluent with the product.
The catalyst for an oxygenate to olefin reaction is typically a molecular sieve catalyst. It is formed into catalyst particles. The presence of the catalyst fines and large quantities of water make removal and disposal of both the water and catalyst fines a unique problem in the oxygenate to olefin process. The solution envisioned and provided by the present invention to this catalyst loss problem involves the use of a wet scrubbing step designed to recover substantially all of the product effluent contaminating catalyst particles and to provide a slurry of these catalyst particles in a scrubbing liquid such as water with subsequent recycle of at least a portion of the catalyst particles contained in the resulting slurry to the OTO conversion zone or to the associated deactivated OTO catalyst regeneration zone thereby recapturing the catalytic activity of these contaminating catalyst particles and diminishing the need for adding fresh catalyst to make-up for catalyst losses.
U.S. Pat. No. 6,403,854 describes a two-stage quench for use with the oxygenate conversion process. The first stage quench removes catalyst fines.
US 2004/0064006 A1 discloses a process for removal of catalyst fines using recycled quench streams. However, in the process taught therein, the recycled quench water will become more and more acidic over time, resulting either in damage to surfaces exposed to this recycled quench water or the need to utilize more expensive stainless steel metallurgy in any section of the equipment exposed to the acidic recycled quench water. Therefore, it would be desirable to have a process for the disposal and handling of catalyst fines that improves process efficiency while preventing the buildup of acids within the recycled quench water.